Behentrimonium chloride (N,N,N-trimethyldocosan-1-aminium chloride; CAS# 17301-53-0), a compound commonly found in cosmetics such as conditioners, shampoos, and also in household detergents, has the structure shown in 1.

Behentrimonium chloride is widely used in hair products because of its potent conditioning antistatic and detangling properties, its ability to restore and rebuild damaged hair, its softening effects, and its antimicrobial properties.
Behentrimonium chloride is a waxy compound that is generally solubilized in hot water (85° C.). It is used at concentrations between 0.5-3 wt %. At room temperature an aqueous solution of behentrimonium chloride clouds, forming micelles, as would be expected of a cationic long chain alkyl compound. The solution is uniformly cloudy, and becomes increasingly more so as the concentration of behentrimonium chloride increases. Dilution of a concentrated aqueous dispersion of behentrimonium chloride creates a less dilute but homogeneous dispersion of the compound.
Behentrimonium chloride is believed to achieve its beneficial effects on hair in part through its penetration into the hair shaft. Penetration is dependent on the local concentration of behentrimonium chloride on the hair shaft surface. In the application of a product to wet hair, as in the application of a conditioner following the use of a shampoo, dilution on the surface of hair is naturally expected to occur. As the beneficial effect of a compound is dependent on the surface concentration of the compound, then its dilution on the hair surface reduces its efficacy, since diffusion into the hair shaft is a concentration-dependent process.
This could be solved in part by raising the concentration of behentrimonium chloride in the formulation to account for surface dilution. However, in the case of behentrimonium chloride at high concentrations (>7%) the solution becomes viscous, waxy, and solidifies on storage. In addition, as larger amounts of behentrimonium chloride are applied to the hair surface the hair becomes waxy and stiff.
Current hair conditioning formulations that utilize behentrimonium chloride include it as a minor component in a complex mixture that generally includes emulsifying aides such as medium chain fatty alcohols, such as cetyl alcohol, emulsifying wax and silicones, such as dimethicone. These formulations are designed to create a homogenous stable emulsion that coats the hair with a combination of substances, including a low concentration of behentrimonium chloride, that together create the desired effects. While meeting with some success in improving the stability of behentrimonium chloride-containing emulsions, the conditioners are generally oily and deposit an oily residue on the hair.